By far the easiest way that I have found to unlock this achievement is to kill everyone in an Outpost using berserk darts.
Firstly, you want to find an Outpost. These are locations on your map indicated by a small hut; if you see one of those on your map, then you will find an Outpost. One of the easier ones (i.e. you only have to get one key) can be found at Vallee Verte in River Valley - co-ordinates (939,-601). This is the Outpost you attack during Sequence 3, Memory 3.
The best place I have found however, is at Lewisporte in Fort Baie Rouge in the North Atlantic. It has a very good hiding place in some bushes on a cliff overlooking the Outpost - co-ordinates: (725,-385). Check the video below for visual reference.
To complete an Outpost, you have to find the keys to a warehouse (or warehouses, depending on the difficulty). They are held by a particular individual at each Outpost, and you must kill him and loot the body to get it. Once you have the key, unlock the warehouse to claim the goods. Then repeat the same process if you have to find more than one key (it can be up to three).
First you want to find a way in - each Outpost is enclosed, but can be accessed by finding a tree or something similar to gain access. Once you are in, you want to immediately make your way to the nearest patch of bushes - otherwise you will be detected very soon. Once there, you will be relatively safe.
Now you need to start picking off enemies in the Outpost using the berserk darts. As a priority, you want to aim for the snipers - they can be identified as a larger arrow with a reticule on the minimap and will usually be in guard towers or on high walls. There are two reasons for this - one: they are usually the first to notice you when you move around, due to their high vantage point, and two: they have guns, and will likely take out a decent number of enemy guards before succumbing to the poison in the berserk dart.
Once you have killed all the snipers you can see, continue shooting the rest of the enemies, remaining in the bushes. Chances are, in the process of doing this, the guard holding the key will emerge, and either be killed by a berserk guard, or by one of your berserk darts. If there are still enemies in the outpost, but you cannot see them, then move to the next nearest patch of bushes and kill them from there. Outposts tend to be full of these patches, so move between them as you need to. Do not go for the key unless you are sure the area is clear. As a side one - the guard with the key shows up on the mini-map, even after he has been killed.
If at any point, the indicator at the top left of screen, or the circle above any of the guards heads turns red, then you have been detected. I would suggest completing the outpost as they take a while to reset, and you will only be costing yourself supplies.
Once all the guards in the Outpost are killed, you are safe to go an loot the body (or bodies if there are multiple warehouses). Unlock the warehouse(s) and take the loot. Once you leave the Outpost, the achievement will unlock.
The video below shows how I went about unlocking this achievement at Lewisporte in Fort Baie Rouge. It has two warehouses, but the process is exactly same as for one warehouse and the rewards are greater.